This invention pertains to the detection of defects in lithography masks, particularly in x-ray lithography masks.
The linewidth of circuit elements in integrated circuits has decreased considerably in recent years; the current state of the art is a linewidth of about 1 micron, and the state of the art may reach linewidths of 0.25 to 0.5 micron within the next five years. Optical techniques for producing circuit elements with such narrow linewidths encounter serious difficulties from diffracton effects.
X-ray lithography has been used to make patterns with narrower linewidths than are possible with optical techniques. The optimum x-ray wavelength for such lithography has been found to be about 10 Angstroms.
Defects on the masks used for such lithography are a serious problem. The present state of the art in producing adequately defect-free masks is poor, being about 30 defects per cm.sup.2. (A defect is a pinhole, an opaque area, or other irregularity of a size at least on the order of 1/4 the linewidth of the circuit.) Because an integrated circuit may have 10 levels on a 1 cm.sup.2 chip, there should ideally be no more than 0.1 defect per cm.sup.2 on the mask.
Thus there exists a need for more defect-free masks, a need to repair masks which have defects, and there particularly exists a need for an improved method of inspecting x-ray masks to identify and locate defects.
Inspection serves at least three functions: (1) to improve process control in making masks, (2) to identify defects on a mask for making repairs, and (3) to qualify masks before use in manufacturing chips.
There are two principal types of mask defects: (1) a spot, an area that absorbs radiation where it should not, and (2) a hole, an area that does not absorb radiation where it should. These defects may lie in unpatterned areas, patterned areas, or on feature edges. There can also be hybrids or combinations of these two principal types of defects. An apparent optical defect may not be an x-ray defect. For example, a dust speck on a mask may be optically visible but x-ray transparent. Similarly, an x-ray defect may not be apparent optically.
Other than direct optical inspection, which is lengthy, tedious, and inaccurate, there have been at least three prior approaches to x-ray mask inspection. The first is inspection of the mask with an electron beam in reflection or transmission. The response of a mask to electrons is not identical to its response to x-rays, however, and data rates are slow--an inspection could, in principle, take weeks.
The second, and more commonly used, prior method of mask inspection is to make a copy with the mask, and to inspect the copy. This technique is exemplified by Hazama, U.S. Pat. No. 4,718,767. Typically a copy of the mask will be made on a transparent wafer coated with a photosensitive layer, and after development the resulting pattern on the wafer will be optically inspected. Existing optical inspection techniques pass light, typically blue light of wavelength 0.4 or 0.5 micron, through the printed pattern onto an electronic detector, and a comparison of the resulting signal is made either to a software representation of what the circuit ideally should be, or to a comparable signal from a second, ostensibly identical mask. Alternatively, typical characteristics of printed defects, such as improper linewidth or edge characteristics, are extracted from the signal. However, it is difficult using this technology to detect defects smaller than about 1/2 to 1 micron, both because of the wavelength of the light used in inspection, and because of the complexity of observing and separating the defect from the larger pattern in which the defect lies.
The third prior method of mask inspection is that of Tanimoto, U.S. Pat. No. 4,586,822. This reference discloses exposing a photosensitive layer on a wafer through a first positive mask, and then through a first negative mask of the same circuit or pattern. Each mask is carefully aligned in the same position, so that if a positive resist on the wafer is used, after development the wafer will have only islands of resist remaining, corresponding to areas which are opaque in both masks. Thus some of these islands correspond to spots in the first positive mask, but holes in that mask are not detected. This reference also discloses a method of detecting holes in the first positive mask, by replicating the entire pattern of the first positive mask onto a negative photoresist, and creating a second negative mask which is a negative of the first positive mask. Similarly, a second positive mask is created which is a negative of the first negative mask. The second positive and negative masks are then printed on a resist as the first masks were, and after development islands of resist will remain, some of which will correspond to spots in the second negative mask, some of which in turn will correspond to holes in the first positive mask. The islands of resist remaining are detected by the scattering of laser light in a dark field. The scattering will detect edges of the islands, but will not directly detect interiors of larger islands. The technique of this reference has the disadvantages (1) that extra masks must be generated in addition to those to be used directly in actual printing, and (2) that spurious results may arise--some new defects may be introduced in the extra copying steps, and some defects may not be completely copied in the extra copying steps, and therefore may not be detected.
Pilkington, U.S. Pat. No. 1,135,919 discloses a method for detecting forgeries by projecting a negative image of a suspected forgery onto a standard positive enlargement, and observing light areas, which would indicate distortions. This technique, from a nonanalogous art, was designed primarily to detect distortions in forgeries rather than small defects; is not directly usable in lithography; does not use a photosensitive material in the detection step per se; would not identify distortions corresponding to holes in the original, positive, suspected forgery; and requires the creation of extra, intermediate images not otherwise usable, allowing the possibility of creating new, spurious errors in that intermediate creation step.